Hello there, @sue40.
I want to make sure that you'll be able to access QuickBooks and make negative VAT adjustments on your most convenient browser.
In cases line this, I'd recommend clearing your browser's cache. Web-based platforms like QBO collects cache files (cookies) to save you time when loading repetitive data and images. These files can often become outdated or corrupted, causing issues like the one you've explained.
Clearing them out usually resolves such errors, and the links below will help you accomplish that in all supported browsers:
By doing this, you'll QBO will be refreshed and you can now start making your adjustments.
I've also added here a link that you can visit about the browser that works best with QuickBooks Online: System Requirements for QuickBooks Online. Furthermore, you may scan through this article about VAT adjustments for your future reference: Adjust your VAT liability
This should get you covered today.
Don't hesitate to reach back out to me if there's anything else you need about this concern. I'd be pleased to help. Take care!